A handful of 2020 Democratic presidential candidates publicly released their tax returns on Monday, attempting to contrast themselves with President Donald Trump, who has withheld his private financial information for his entire presidency, even in the wake of congressional investigations.

But many of those Democratic candidates' tax returns revealed how little they give to charity each year compared with average American households with similar income levels. This is despite the significant wealth of those presidential candidates.

Former Texas Rep. Beto O'Rourke released 10 years of tax returns filed jointly with his wife, Amy. In 2017, the O'Rourkes earned $366,455 but gave only $1,166 to charity. That is just over 0.3% of their income in charitable contributions. Similarly, the O'Rourkes gave just 0.2% of their income to charity in 2016.

By comparison, the average American household in the same income bracket that itemizes its deductions gave $21,364 to charity in 2016, according to IRS data.

The O'Rourkes' charitable contributions do not get much larger in other years either. The last time they gave more than $10,000 was in 2013, when they donated just over 4% of their income to charity.

Like O'Rourke, Vermont Sen. Bernie Sanders gave relatively little to charity compared with other Americans, despite writing a book that made him a millionaire in 2016.

Sanders, who for years resisted releasing several years of his tax returns, earned $1,073,333 but gave $10,600 to charity — or just less than 1% of his income. Based on 2014 IRS data, people who made more than $1 million that year took an average charitable deduction of $382,953, or 5.6% of their income.

Sanders' campaign said that proceeds from his 2012 book, "The Speech," are donated to charity and not reflected in his returns.

Sanders boosted his charitable giving in the years since 2016, giving 3.4% of his adjusted gross income in 2018 and 3% in 2017.

Other candidates also donated relatively small amounts to charity, including Sen. Kirsten Gillibrand of New York, who along with her husband donated less than 2% of their $215,000 income to charity last year. Minnesota Sen. Amy Klobuchar and her husband gave $6,600 of their $338,500 to charity, a percentage similar to Gillibrand's.

California Sen. Kamala Harris gave 1.4% of her and her husband's $1.9 million income to charity in 2017. And during several years of her time as California's attorney general, Harris reported no charitable donations.

Higher charitable contributions came from Democrats such as Washington Gov. Jay Inslee and Massachusetts Sen. Elizabeth Warren. Inslee and his wife donated 4% of their income in 2017, while Warren and her husband contributed 5.5% of their $906,000.

Sen. Cory Booker, a Democrat from New Jersey was the lone standout among the 2020 Democratic candidates so far.

Booker released 10 years of tax returns on Wednesday evening. In 2018, he made $152,639 (his US Senate salary); he paid $22,781 in federal taxes (a 19% tax rate); and he donated $24,000 to charity. His charitable donation amount equals a little more than 15% of his 2018 income, and it makes him the Democratic candidate who gave the largest monetary percentage of his salary to charity.

A lack of charitable contributions has been a sore spot for presidential candidates in the past. In 2012, Republican presidential nominee Mitt Romney and then-President Barack Obama each gave considerably more to charity than O'Rourke and Sanders, donating 29% and 22%, respectively.

But also in 2012, former Vice President Joe Biden — now a potential 2020 presidential candidate — gave just 1.5% of his income to charity along with his wife, Jill Biden. The small portion the couple gave to charity was widely criticized, but it was larger than O'Rourke's and Sanders' 2016 donations.

Other tax-return releases from the Obama campaign also showed how the Bidens donated $3,690 to charity over the course of an entire decade.

Meanwhile, Trump has refused to release his tax returns, even after demands from the Democratic-controlled House Ways and Means Committee, setting up a long legal fight between the president and Congress.